The present invention relates to ferromagnetic thin-film memories and, more particularly, to ferromagnetic thin-film memories in which states of the memory cells based on magnetization direction are determined through magnetoresistive properties of the thin-film sensed by an electronic circuit.
Digital memories of various kinds are used extensively in computers and computer system components, digital processing systems, and the like. Such memories can be formed, to considerable advantage, based on the storage of digital system bits as alternative states of magnetization in magnetic materials in each memory cell, typically ferromagnetic thin-film materials. The information stored in such films is stored therein through selected directions of magnetization occurring in that film, this information being obtainable from such films through either inductive sensing to determine the magnetization state, or by magnetoresistive sensing of such states. Ferromagnetic thin-film memories of this nature may be conveniently provided on the surface of a monolithic integrated circuit chip to provide easy electrical interconnections between the memory cells and the memory operating circuitry provided in the chip.
Ferromagnetic thin-film memory cells can be made very small and they can be packed very closely together to achieve a significant density of stored information bits, properties which are the basis of permitting them to be provided on the surface of a monolithic integrated circuit, as indicated above. Suitable constructions for such cells can be found in U.S. Pat. No. 4,731,757 to Daughton et al entitled "Magnetoresistive Memory Including Thin Film Storage Cells having Tapered Ends" and U.S. Pat. No. 4,780,848 to Daughton et al entitled "Magnetic Memory" and an earlier filed application by J. M. Daughton having Ser. No. 07/161,534 entitled "Magnetic Device Integrated Circuit Interconnection System," all of which have been assigned to the same assignee as the present application and are herein incorporated by reference. A structure analogous in some respects to those disclosed there is shown, as an example, in FIG. 1.
There, a bit structure, 10, from a series string of them in a monolithic integrated circuit is presented in a portion of that integrated circuit such that the opposite ends thereof, 11, are exposed. Bit structure 10 is shown formed over a semiconduotor material body, 12, in and on which the monolithic integrated circuit is formed. Bit structure 10 is provided directly on an insulating layer, 13, supported on a major surface of body 12 in the integrated circuit chip. As indicated above, just a portion of the integrated circuit is shown, and then only a small part of the semiconductor body for that portion of the integrated circuit is shown in the figure.
Semiconductor material body 12 is typically formed of a doped silicon material of primarily either p- or n-type conductivity, with other regions of opposite conductivity provided therein to form circuit elements or portions of circuit elements. Insulating layer 13 is typically formed of silicon nitride both as an electrical insulator and as an oxygen barrier to prevent any oxygen from migrating therefrom into bit structure 10 and oxidizing any of the magnetic materials therein.
Bit structures like bit structure 10 are usually provided, as previously indicated, in a memory as a series string of such bit structures extending both ways from ends 11 where exposed at the edges of the integrated circuit portion shown in FIG. 1. The bit structures in series are connected to each other in such an arrangement at junctures, 14, provided at each end of each of the bit structures where they provide an electrical short circuit for interconnecting them, and at the ends of each series string for interconnecting it to other circuit components in the monolithic integrated circuit operating the memory.
The remainder of bit structure 10 disposed on the exposed major surface of insulating layer 13 is comprised of a lower ferromagnetic thin-film, 15, and an upper ferromagnetic thin-film, 16. Ferromagnetic thin-film layers 15 and 16 exhibit uniaxial anisotropy, magnetoresistance, little magnetostriction, and are of an alloy composition typically comprising nickel, cobalt and iron. Typically, proportions of each are chosen to strongly reduce or eliminate any magnetostrictive effects in the films, and to improve certain other properties of them for the application.
Between ferromagnetic thin-film layers 15 and 16 is a further thin layer, 17, which usually would not be one exhibiting ferromagnetism, but may be either an electrical conductor or an electrical insulator. Layer 17 must, however, in this construction, prevent the exchange interaction between electron spins on neighboring atoms from coupling across the separation between layers 15 and 16 to thereby lock together the magnetizations of each. A typical choice for layer 17 would be tantalum doped with nitrogen.
A further diffusion barrier and protective layer, 18, is provided over upper ferromagnetic thin-film 16. This layer again can be formed of nitrogen doped tantalum. In junctures 14 there are further provided two other conductive layers, the first layer, 19, being formed of copper doped aluminum on which is provided a second layer, 20, of tungsten. These layers provide an electrical short between adjacent bit structures as indicated above.
An insulating layer, 21, is then provided to electrically isolate bit structure 10 from further structures to be provided thereabove. Layer 21 is typically formed of silicon nitride. A word line conductor, 22, is then provided over layer 21 extending to the tapered portions at the ends of bit structure 10. Both layers 21 and 22 are only partially shown in FIG. 1, with other portions thereof having been removed from the right-hand portion of bit structure 10 in that figure so that bit structure 10 can be more clearly seen. Word line conductor 22 is typically formed with a thin layer of titanium and tungsten followed by a layer of copper doped aluminum. A passivation layer, 23, of silicon nitride is then provided over the entire structure, though this layer is also only partly shown, to protect it from external contaminants.
Bit structure 10 can be operated in a longitudinal mode having its easy axis extend between junctures 14 perpendicular to the direction of extension of word line conductor 22, or in a transverse mode having its easy axis of magnetization parallel with the direction of extension of word line conductor 22. In either situation, information, kept as a binary bit having one of two alternative logic values in bit structure 10, is stored therein in layers 15 and 16 by having the magnetization oriented to point in one direction or the other (but opposite in each of these layers to the direction in the other), generally along the easy axis of magnetization. If the direction of magnetization is caused to rotate from such a direction by external magnetic fields, the electrical resistance of layers 15 and 16 change with this magnetization direction rotation because of the magnetoresistive properties of such layers.
Near the edges of films 15 and 16, assuming that the easy axis is parallel with word line 22, anisotropy fields are dominated by the demagnetization fields due to the "free poles" at these edges. If the magnetizations of films 15 and 16 were saturated, the demagnetization fields would approach half the saturation fields, or perhaps 5,000 Oersteds, for these films with the alloys described here. Typical films of these alloys will have a coercivity and an anisotropy field on the order of only 20 Oersteds, leading to instabilities in the magnetization at the edges of these films.
In such large demagnetization fields, electron spins at the edges of the films are constrained to lie nearly parallel to these edges and the direction of elongation of these films. The directions of these electron spins only gradually turn to pointing across the films further inward toward the center of the films with the demagnetizing fields no longer overcoming the isotropy field. The rate, shape and distance of the currents are all a complex function depending on the magnetostatics of the situation, the quantum exchange interaction between adjacent atom electron spins, and the anisotropy considerations not unlike those leading to Neel walls.
Thus, the magnetization along the transverse axis is at or very nearly at zero at the outer edges of the films where word line 22 crosses them, and gradually increases in inward locations which are closer to the interior of the films, the magnetization value increasing toward the saturation value occurring in the central portions of the films. In the regions between the portions adjacent to the exterior edges of the film and the point where magnetic saturation begins, the film magnetizations are in transition from pointing along the direction of elongation to pointing along the direction of the easy axis. Therefore, such ferromagnetic thin-films having their easy axes extending in directions transverse to the direction of elongation thereof, or parallel to word line 22, do not truly saturate across the films along the easy axis but only across a part of such a film.
A representation of the magnetizations of a section of one of films 15 and 16 from between its tapered parts is shown in FIGS. 2A and 2B. FIG. 2A shows the magnetizations for the storage of a "0" logic value bit of information with the magnetizations at central locations shown pointed upward in that figure and edge magnetizations therein shown pointed primarily to the right. FIG. 2B shows the magnetizations for the storage of a "1" logic value bit of information with the magnetizations at central locations shown pointed downward in that figure, and with edge magnetizations therein shown still pointed primarily to the right.
Sense current refers to the current flow through bit structure 10 from one juncture 14 to the other juncture 14 thereof, and word current refers to current flowing in word line 22 adjacent to, and extending in a direction traverse to the direction of elongation of, bit structure 10. Bit structure 10 can be placed in one of two possible magnetization states along an easy axis thereof as shown in FIGS. 2A and 2B through the selective application of sense and word currents, i.e. information can be stored or "written" into bit structure 10.
Bit structure 10, in the configuration shown in FIG. 1 or in other typical configurations, in the past was typically placed in a "0" logic value magnetization state on the basis of providing a word current of typically 10.0 mA to 30.0 mA flowing in a direction which creates a magnetic field in films 15 and 16 oriented in the same common direction as the edge magnetizations of those films were primarily. A current through word line 22 giving such a result in a magnetic field will be termed a positive word line current or positive word current. Previous to this current flow, the edge magnetizations of films 15 and 16 were set by the application of a strong external magnetic field of typically a few thousand Gauss. The setting of a "0" logic value magnetization state was then completed by providing a sense current of typically 2.0 mA to 3.5 mA coincidentally with the provision of the word current.
The opposite magnetization state representing a "1" logic value magnetization state was alternatively set into bit structure 10 through providing the same word current in the same direction, i.e. a positive word current, but providing a sense current of the same magnitude in the opposite direction through bit structure 10. Such magnetization state changes will occur very quickly after the proper current levels are reached, such changes between such states occurring in less than about 10 ns.
Determining which magnetization state occurs in bit structure 10, i.e. retrieving or "reading" the information stored in bit structure 10, is done by providing externally caused magnetic fields in that bit structure through providing coincidental sense and word currents to rotate the magnetization of that structure. As indicated above, differences occur in the electrical resistance encountered between junctures 14 in bit structure 10 for the magnetization being in different directions in the structure, including changing from one easy axis direction magnetization state to the opposite direction state. These differences in electrical resistance of bit structure 10, due to its magnetoresistive properties, result if differences in direction occur between the orientation substantially of magnetization in the thin-films and the direction of current being passed therethrough. The maximum resistance occurs when substantially the magnetization direction in the film and the current direction are parallel, while the minimum resistance occurs when they are perpendicular to one another.
The electrical resistance of the magnetoresistive resistors, or films 15 and 16, forming bit structure 10 can be shown to be a constant value representing the minimum electrical resistance encountered plus an additional value depending on the angle between the direction of current in the thin-films and the magnetization primarily occurring therein. This additional resistance has a value which follows the square of the cosine of that angle. As a result, there will be differences in the voltage developed across bit structure 10 between junctures 14 by a constant value sense current flowing therethrough depending on the predominant magnetization direction in this structure, and so depending on which magnetization state initially is present therein.
In an information retrieval operation of the kind that has been used in the past with a bit structure such as bit structure 10, a current through word line 22 is again applied which results in a magnetic field in layers 15 and 16. The current flow direction is chosen such that this resulting field is directed parallel with the common direction predominantly followed by the edge magnetizations of those layers as previously set by an external magnetic field as described above, that is, a positive word current. This word current for this method of information retrieval can be a current of a value comparable to that used in storing information, as described above, if destruction of the information content of bit structure 10 is an acceptable result of such a retrieval. Otherwise, the word current must be of a significantly smaller magnitude if such a destruction of information is to be avoided.
However, the choice of such a smaller current will lead to a smaller resistance difference between "0" and "1" logic value magnetization states and so a smaller output signal. A word current in the writing range of from 10.0 mA to 30.0 mA will lead to either a continued "0" state, or to an initial "1" state becoming thereafter, as a result of such an information retrieval, a "0" state. On the other hand, the advantage to providing larger word current and risking such a destruction of information is that this will result in the maximum resistance difference between the magnetization states and so the largest output signal. Typically, this output signal is provided through the passing of a sense current through bit structure 10 between junctures 14 in the same direction as it was provided for the establishment of a "0" magnetization state in the bit structure.
An example of the retrieval situation for this method is given in FIG. 3 based on the "0" and "1" logic value magnetization states electrical resistance characteristics of bit structure 10 shown there. Bit structure 10 in the "0" magnetization state exhibits the lower curve in FIG. 3 as its electrical resistance versus applied word current characteristic, and is shown there for a sense current of 3.5 mA. This lower curve, or "0" magnetization state resistance characteristic, can be seen to have a generally positive slope. The upper curve is the electrical resistance characteristic of bit structure 10 with the same sense current but which has, instead, a "1" magnetization state written therein.
In this latter situation of a stored "1" magnetization state, bit structure 10 exhibits a higher electrical resistance versus word line current than it does in the "0" state, at least for lower word line current values up to a break point. This situation is shown in the higher valued and more positively sloped resistance characteristic represented in the upper curve of FIG. 3. For word currents sufficient to pass the break point, the "1" magnetization state is switched, or "rewritten," to a "0" magnetization state just as in the storing, or "writing," of selected magnetization states in bit structure 10 described above. This causes the resistance characteristic to revert to essentially that of the lower curve in FIG. 3.
Thus, if the word current is chosen to be sufficient to change a "1" logic value magnetization state to a "0" magnetization state, the information retrieval process will be a destructive one leading to the loss of the information stored in bit structure 10 (whenever a "1" state is in storage at the start of the process). Such an information loss would have to be replaced in a subsequent refresh writing operation if information integrity in the memory array using such bit structures was to be maintained.
On the other hand, as was seen in FIG. 3, the maximum resistance change occurs at the break point giving the peak resistance change in response to a word current, and so the peak voltage change across bit structure 10 between junctures 14 for a given sense current flowing therethrough. This is the maximum signal available for indicating the presence of a "1" logic value magnetization state as opposed to a "0" magnetization state, and is typically 1.0 to 2.0 mV in this information retrieval method for a 3.5 mA sense current. The application of word currents less than the break point word current value in FIG. 3 will still provide a voltage change across bit structure 10 in response to a sense current therethrough, but of a magnitude less than the peak signal available. Such a nondestructive information retrieval, of course, eliminates the need for a later refresh operation.
The arrows shown along the two resistance characteristics in FIG. 3 are those showing the resistance change locus which will occur in providing a word line current pulse going from 10.0 mA to 30.0 mA and back to 10.0 mA. The short arrows along the lower current characteristic give the electrical resistance change locus for there being a "0" magnetization state in bit structure 1-0 at the time of such a word current excursion. The long arrows along the upper characteristic, and along the lower characteristic where they merge, gives the results for the occurrence of a "1" magnetization state in that structure during the same word current excursion. The dashed arrows along the upper characteristic show the resistance path taken for an alternative word current excursion with a "1" magnetization state in bit structure 10, a word current excursion in which the magnitude thereof is limited to reaching a value less than the break point shown there.
The change in the electrical resistance values in FIG. 3 are shown to be in tenths of Ohms in a design for bit structure 10 having around 100 .OMEGA. or so of total electrical resistance between its junctures 14. A 0.3 .OMEGA. change with a sense current of 3.5 mA flowing through bit structure 10 means that the voltage change peak is only on the order of 1.0 mV in a nominal voltage value across that structure of 0.3 V.
Such changes in the voltage drop across bit structure 10, or equivalently, in the electrical resistance of that bit structure, mean a very small output signal must be detected on top of a relatively very large nominal value, and typically in the presence of an electrically noisy background. Such conditions can be overcome to some extent by sensing the output signal from bit structure 10 in a differential arrangement with a reference voltage. This arrangement, in effect, more or less subtracts out that large underlying voltage leaving the increment due to the presence of a word line current being applied to bit structure 10.
However, differences between various bit structures in which information is stored in a memory lead to the underlying voltage occurring across a bit structure 10 due to the sense current therethrough to differ from one bit structure to another. Therefore, choosing a reference value for use in the differential sensing system, to be subtracted from the voltage across bit structure 10 in which the information to be retrieved is stored, would be difficult if a single constant value is to be chosen. In addition, conditions such as temperature, differing positions of bit structures in the array, and others will also vary from one bit structure to another leading to further differences in the underlying voltage occurring across each. These differences can be compensated for to a significant extent by the use of other, similar bit structures as resistor references in a reference arrangement which results in them having the same sense current flow therethrough but which are not subjected to a word line current flowing thereover.
Further steps can be taken to enhance the detection of the signal in a bit structure 10 by limiting sources of errors such as offsets in the electronics between an information content bit structure and the reference bit structure it is being differentially sensed against. This can be done through use of "autozero" techniques in which capacitors are used to couple a bit structure 10 and an associated reference bit structure to a differential sensing amplifier. These capacitors are initially charged to the different voltages across the bit structure arrangements connected to each amplifier input to thereby eliminate offsets from being applied to that amplifier during a subsequent information retrieval operation. Further, the amplifier can be bandwidth limited to reduce the amount of electrical noise transmitted therethrough. These techniques are more fully described in U.S. Pat. No. 4,829,476 to DuPuis et al which is assigned to the same assignee as the present application and incorporated herein by reference.
Despite such measures, none of them alone or in combination with others are sufficient to eliminate errors in detection of the output signals of bit structures like bit structure 10 in all circumstances. Thus, there is a strong desire to provide an increased output signal to reduce such detection errors.